GLASS CONVERTIBLE TO TRANSPARENT GLASS CERAMICS CONTAINING AlPO{11

ABSTRACT

Glass ceramics made from an initial glass composition, in weight percent, of SiO2 : 35-70 Al2O3 : 17-32 P2O5 : 5-17 Li2O : 2-5 MgO : 0-4 ZnO : 0-5 TiO2 : 1.5-6 ZrO2 : 0.5-3 Na2O : traces-0.6 As2O3 : 0.5-1.0, where the P2O5 to TiO2 ratio is up to about 4, and where the sum of ZrO2 and TiO2 is at least about 3 weight percent.

United States Patent Petzold et a1.

[ Feb. 15,1972

1541 GLASS CONVERTIBLE TO TRANSPARENT GLASS CERAMICS CONTAINING ALPO [72] Inventors: Jurgen Pelmld; Herwig Scheidler, both of Mainz-Mombach, Germany I73] Assignee: JENAer Glaswerk Schott & Gem,

Mainz/Hattenbergstrasse, Germany [22] Filed: July 1, 1968 2| 1 Appl. No.: 743,550

[30] Foreign Application Priority Data July 1, 1967 Germany ..Pl5 96 860.2 Nov. 22, 1967 Germany... ..P15 96 865.7 Jan. 3, 1968 Germany ..P l6 96 062.6

[52] U.S.C1.... 106/39 DV, 106/52, 65/33 FOREIGN PATENTS OR APPLICATIONS 6,605,388 10/1967 Netherlands ..l06/39 OTHER PUBLICATIONS Perrottu et aL-Beta Eucryptite Crystalline Solutions Involving P 5+" J. Am. Cer. Soc. Vol. 50,N0. 2,p. 1 12. Feb. 21, I967 Primary Examiner-James E. Poer Assistant Examiner-W. R. Satterfield AttorneyBurgess, Dinklage & Sprung [57] ABSTRACT Glass ceramics made from an initial glass composition, in weight percent, of

SiO: I 35-70 20 12-32 P 0 5-17 L130 2-5 MgO 0-4 ZnO 0-5 T102 |.5-6 ZrO 0.5-3 N'nlgO traces-0.6 A3 0,, 0.5-1.0,

where the P 0 to TiO, ratio is up to about 4. and where the sum of ZrQ, and TiO is at least about 3 weight percent.

13 Claims, No Drawings GLASS CONVERTIBLE TO TRANSPARENT GLASS CERAMICS CONTAINING ALPO,

The present invention relates to transparent glass ceramics having low coefficients of thermal expansion. It more particularly refers to transparent glass ceramic articles having coeffi- 5 cients of expansion of about zero, as well as to methods of producing such articles.

The basis for the method of converting glasses by controlled crystallization into vitreous-crystalline condition and thus obtain glass ceramics, the properties of which are determined eslo sentially by the crystal phase distributed at random in the glass matrix, have already frequently been described in detail in the literature (German Pat. No. 1,045,056; P. W. McMillan, Glass Ceramics," Academic Press, London and New York, 1964).

Transparent glass ceramics of low thermal expansion are known, whose low a values are caused by solid solutions contained therein having an B-quartz structure (frequently also referred to as solid solutions with B-eucryptite structure or {3- eucryptite like crystals). Such glass ceramics are obtained by controlled crystallization of glasses of the system LEO-A1 0,- SiO with T10, and/or ZrO as nucleating agents. It is furthermore known that transparent glass ceramics with B-quartz can be obtained from base glasses, which, in addition to Li O, A1 0 SiO 'l'iO and ZrO also contain specific amounts of MgO and ZnO. The components MgO and ZnO, in case of a suitable ratio of Li,O+MgO+ZnO/AI O enter into the solid solutions with quartz structure. The concentration range of glasses, from which fil-quartz solid solutions can separate out so that transparent glass ceramic materials of low thermal expansion are produced, has been considerably broadened by the components MgO and ZnO. Nevertheless, such glassceramic starting glasses, having the main components SiO,, A1 0,, Li O, MgO, ZnO, T10 and ZrO,, have high working temperatures ('rp=l(l poise), so that difficulties are encountered in attempting to use continuous working processes (au tomatic pressing or blowing) for such glasses.

The object of the present invention is to develop glasses having viscosity properties which make it possible both to 40 press them automatically and to blow them automatically in continuous operation, and which can thereupon be converted by controlled crystallization, during an economical, uncomplicated heat treatment, into a vitreous-crystalline condition in such a manner that there are produced objects which are of transparent appearance, have low coefficients of expansion, preferably close to a value of zero, and are not deformed.

Other and additional objects of this invention will become apparent from a consideration of this entire specification, including the claims hereof.

In accord with, and fulfilling these objects, one aspect of this invention resides in glasses of the following compositions in weight percent.

wherein the ratio of P 0 to TiO is up to about 4, and wherein the sum ofZrO and TiO is at least 3 weight percent.

1n a preferred aspect of this invention, glass compositions are provided containing at least about 90 weight percent of solid solutions ofquartz structure having the general formula Li ,Mg,.Zn 'O'AI O XA1POAy2x)Si0 wherein: V=OCI 4 The components Na O (as nitrate) and As,0, serve as fining agents. Glass-ceramic starting glasses of the composition range in accordance with the invention have working temperatures (Tat 1p=10 poise) of between l,ll ,320' C. and are thus easy to adapt to the traditional processes of working. such as blowing, pressing, rolling, and casting. In order to convert the articles produced from the starting glasses in accordance with the invention into transparent, vitreous-crystalline condition, in which condition they do not deform to any appreciable extent, they are heated at most about 3" C./min. to temperatures of 50-70 C. above and transformation points of the glasses, held there for at least about 30 minutes to develop the crystallization nuclei, heated at a rate of not more than about 3 C./min. to temperature of l40-l90 C. above the transformation points of the glasses, maintained at such temperature until sufficient crystallization is obtained, and then cooled as rapidly as desired to room temperature. The resultant glass-ceramic bodies are transparent, of yellowish-brown color, have a values of between 6 and x35 10 C. and have better mechanical and chemical properties than the initial glasses which have been thus heat treated.

The low viscosity of the glasses of the invention, as compared with known starting glasses for transparent glass ceramics of low thermal expansion, was essentially obtained by reduction of the SiO, content and with an increase in the A1 0 and P 0, contents in the molar ratio of about 1:1. The fact that it is nevertheless possible to convert such glasses, which may contain as little as 35 percent by weight SiO in addition to as much as 17 percent by weight P 0 by crystallization of B-quartz solid solutions into a transparent. vitreouscrystalline condition, with the content of crystalline phase being not lower than in the case of compositions richer in SiO is to be ascribed to the phenomenon, clearly established by X-ray and infrared spectroscopic examinations, that in B- quartz solid solutions Si0 can be extensively replaced isomorphically by AlPO without the characteristic low expansion of the B'quartz solid solutions being lost.

Starting from a solid solution with fi-quartz structure of the system Li O'A1 O '2SiO SiO, with the general form Li O'Al O -ySiO,(y I 2), there is obtained, when the substitutions ZnQAl 2Si; MgQAl'" 2Si"* and P Al Z are carried out, a solid solution with quartz structure of the general formula Li ,-Mg,Zn,,.-O'Al,O 'xAlPO (y2x)SiO This solid solution was used for the calculation of the initial glasses in accordance with the invention, in which connection in each case at least 90 percent by weight of each glass composition corresponds to the composition of a quartz-solid solution of the formula indicated. The remainder is contributed by the components responsible for nucleation (TiO,+ZrO,) and for fining (Na o as nitrate and Asp. or to a limited excess of A1 0 (referred to the solid solution composition). The initial glasses of this composition can be particularly readily converted into the transparent, vitreous-crystalline state by controlled crystallization since it can be assumed that the crystal and residual glass phases contained in the glass ceramics are similar with respect to their chemical composition so that no great differential in the index of refraction is to be expected at the phase boundaries.

By way of explanation, but not limitation, of the present invention, the following 10 examples are set forth in Table l; in Table 2 properties ofthe initial glasses are compared with the properties of the corresponding glass ceramics obtained therefrom.

TABLE 1 611.7 57.4 55.3 53.2 51.3 46.7 46.0 44.5 38..1 19.7 23.3 35 1] 26.3 24.3 28.8 28.8 $1.5 2'9 5 6.3 6.0 7.9 7.8 10.5 11.6 11.6 15.7 15.3 3.1 51 4.5 3. S) 3.4 3.8 3.8 3.1 2.2 0.11 2.7 0.5 1.2 1.0 1.1 1.1] 0.9 2.2 1.8 1.7 1.5 2.2 2.0 2.2 2.2 1.8 4.4 4.5 3.2 2.2 2.6 4.6 3.0 3.7 4 6 -l.ti 1.1 1.7 1.9 1.7 1.8 1.8 1.8 If! 1.. (l5 0.5 0.5 11.3 [1.6 11.5 0.5 0.5 [1 5 11.5 0 fl 0.? 0.5 0.5 11.5 0.5 11.5 0 5 TAB LE 2 \urnber H l 2 3 4 5 6 7 8 J 10 Glass properties:

afn lO p.)( C.) 1.300 1.260 1.240 1,206 1,195 1,226 1.205 Transformation:

Temperature C.) 688 630 660 654 650 639 642 640 637 645 At -300-" C.) 31. 6 38.7 37.7 45.0 42. 0 40.1 43.0 44. 0 35.9 36.6 Density (grjemfi) 2. 45 2. 46 2. 45 2. 47 2. 46 2. 47 2, 49 '2. 45 2. 55 Properties of the transparent glass ceramics at ZOSOO- Cf) 4.6 3.0 12. 0 G. 0 0.5 0. 5 1.0 3. 0 18.0 35. 0 Density (gr/emi t 2.65 2 56 .54 2. 53 2.65 2.54 2.55 2. 57 2.67 Crystal phases Quartz solid solutions, plus ZrO containing nucleus phase it is evident from the examples given that the coefficient of expansion on the resultant transparent glass ceramics depends little on their P,O content. it can easily be controlled by the Li O content. In order to obtain particular low coefficients of expansion with samples having a high amount of P 0 one requires, to be sure, more Li,O than in the case of samples which are poor in P 0 increasing contents of P 0, in the base glasses require higher TiO contents if transparent glass ceramics are to be produced. The ratio of P 0 to TiO must therefore be s 4, in which connection the ZrO, addition should not be less than 0.5 percent by weight. The total amount of nucleation agents should not be less than 3 percent by weight, and the minimum TiO concentration should not be less than l.5 percent by weight ifthe controlled crystallization of the initial glasses is to lead to transparent vitreous-crystalline bodies.

It has furthermore been found that initial glasses in accordance with the invention, having the following composition range, are preferred for automatic pressing in continuous tank operation due to their viscosity properties and the position of their liquidus temperatures:

Composition in percent by weight:

52-5o sio, 24.543 A|,O 7 5-9 5 1 7-4 Li,0 o v-i.i M o 2-2 s ZnO 2 64.9 TiO, I 7-H? zro, o 5-0 1 MP os-oa m c The compositions of these base glasses should correspond to at least 90 percent by weight of the composition of a solid solution with quartz structure of the general formula Li Mg,-Zn,. O-M 0 -.rAlPO.(y-2x)SiO and, aside from an excess of N 0 which is limited with respect to this formula. in addition only contain components for nucleation (TiO t ZrO -l and for fining (Na- O as Na NO- and A5 0,). The conversion of such base glasses into the transparent vitreouscrystalline condition. whereby shaped articles are obtained having coeliicients of expansion, which are at or close to u value of zero, is accomplished by heating them. at a maximum of 3 ('../min.. to about 50 (1 above their transformation temperature; holding them at this temperature for at least about minutes: heating them further. at a rate of not more than .iboilt T (Iimin. to 791) 850 holding them at that temperature until sufficient crystallization has been obtained; and then finally cooling them as rapidly as desired.

This invention is illustrated by the following examples which are in no way limiting thereon.

EXAMPLE l To produce IOU kg. of glass. there are mixed together in a commercial mixer for about 30 minutes 52.28 kg of Sand, 3| 55 kg. hydrated alumina. 9 so kg. of lithium carbonate.

2.24 kg. of zinc oxide, 2.66 kg. of magnesium carbonate, 14.49 kg. of aluminum orthophosphate, 0.50 kg. of arsenic, 1.65 kg. of sodium nitrate, 2.8] kg. of titanium dioxide and 2.77 kg. of zirconium silicate. The mixture is melted down in a ceramic pot, at a melting temperature of L600 C., for l2 hours and refined for l8-24 hours at l,600 C. Glass articles were produced from this melt, by pressing or blowing or pulling or rolling or casting, and finally cooled. The cooling was accomplished from 660 C. with cooling rates of 0.l-20 C./min.. depending on the size and wall thickness ofthe article to be cooled. The properties of this glass. which has the oxide composition set forth below in weight percent, are set forth in Table 3:

5300 $0,; 26.30 AIQO]; 7.80 P 0 l.l0 MgO; 3.85 U 0; 2.20 ZnO; 2.80 TiO [.85 ZrO 0.5 A3 0, and 0.6 N3 0.

TABLE 3 Properties ofth: initial glass Transmitlance A=5SU ml d=l cm )i In order to convert such glass articles, without deformation, into the transparent, vitreous-crystalline state, they are heated at the rate of 2 C/min. to 700 C., maintained at that temperature for 30 minutes, heated at a rate of 2/min. to 8l0 C, held there for 90 minutes. and finally cooled as rapidly as desired A transparent, vitreous-crystalline body produced in this manner has the properties set forth in Table 4.

TABLE 4 Properties of transparent glass ceramics:

Flt-minute to h drolisis DIN n I H cooled. The cooling is effected from 670 C. at cooling rates of O I u (ml-HMO, 0.1 20 per hour. depending on the size and wall thickness on of the article being cooled. The properties of such a starting Resistance glass in accordance with the invention are set forth in Table 5. acid om i2i l6 (ml/m4) TABLE 5 38 Resistance to illk lil Properties of the initial glass DIN 52322 [0 (mg/dm'] 12 Temperature with n==l poise Ci I480 Trunsmllldncr: A= 550 ml d=l cm.) a 70 Temperaww with F poise 1 c l I405 t'i siiilimtphases h-quartz solid Temp rature ith 1= p se 1C i U60 solution Temperature with n=l'l0 ZrO contaIining l5 P J) 1C.) I297 nucleus phase. Trflnll'ormution temperature 660 temperature 1C.) IJlUzH) Coefficient of I expansion X IO'HOJOU C.) t C") 39.5 Furthermore. it has been found that the starting glasses for Dem), (mm 2 producing transparent glass ceramics with coefficients of ex- Knoop hvrdnm a vi (kg/min?) s70 p nsion of about zero in accord with this invention can be aul r ll d ii t r ll bl w witho tdif toma ica ypresse as we as au oma ica y o n Piun.lmmb" 01H ficulty on basis of their viscosity properties and the position of m mndumvii, (kcflyhflmmdd their liquidus temperatures in continuous tank operation if Index ofrefraction l.$2| their compositions lie within the following preferred concenf' t tion ran ein erentb wei ht Trumm'mn" (B550 Ta 8 P v if 8 iiiicimew-s mm.] is) 9| Resistance to 57. SiO, hydrolysis DIN HI I I (mg.Nu,0/g.l U 0| 5 2 i -24 AIIO, Relistance lo .54; PO, acid DIN l2l l6 (mg/11m) 24 3.34.6 up Resistance in l.52 ZiiO alkali DIN 52322 mg/d n qu 0.5-0.9 MgO 2.0-3.0 Tao. i.7-2.u zro M40 ln order to convert such glass bodies, without deformation, os-us hi into the transparent. vitreous-crystalline condition, they were I I I heated, at a rate of 3 C. per minute, first of all to 550 C., and The compositions of the glass ceramics of the invention corh b h 3! a rate of 1 C. per minute to 725 C. held fespondmgt P of least P by 8 to there for 90 minutes, heated further, at a rate of 1 C. per the compositions of solid solutions with quartz structure ofthe 40 minme' m C held there for 180 mimics and finally g f formula w iw cooled as rapidly as desired. The transparent glass ceramic h balanct f from the nucleauo" produced in this manner had the properties set forth in Table ponents (TlOgl'Zl'O-gi, the lining components lNaNO;.+ 6 As O-i). or the M 0 excess limited with respect to the solid solution composition. TABLE 6 The objects produced from glasses of this composition range are heated in accordance with the invention at the rate of at most about 3 C./min. to temperatures of -70 C. Properties ofthe transparent glass ceramic: above their transformation points, held there for at least 30 minutes. heated further at a rate of at most about 3 C./min. to

a a 50 Coefi'icient of temperatures of 815 -850 C.. held there until sufticient "panda" XIUQHHW. I" crystallization has taken place, and finally cooled as rapidly as Coet'ficient of d i d expunlion l0'(20-8OU' C.) 120 C 5 Denlity lg /cm.) 2 52 EXAMPLE 2 Knoop hardneu 50 gr.) "Hm," 1, W6 55 Bending strength (kg/cm 'l 900 An initial glass of the following oxide composition in weight s t. mehed down Poisson: number (i 23K percen Thermal conductivity (ltcal lh m grd i l 40 Index of refraction l 54] SE1 0 SiO l.7 ZnO Abbe number 5 a M10 0'7 0 Tranlmittance (A=55U mit d=5 mm ll?) 7 P,O, Z 9 TD, Resistance to Li,O l.7 ZrO, hydrolysis DIN |2l ll (mg Na,O/g (10H 0.6 Na,0 Resistance to 0.8 ALIOJ acid DIN lllllS (mg /dm '1 l5 Resistance to In order to produce lOO kg. of glass of this composition. 52322 h 57.40 kg. ofsand, 26.77 kg. of hydrated alumina, 8.73 kg. of my lithium carbonate. [.73 kg. of zinc oxide. L69 kg. of magnesi- +ZrO,-containing um carbonate. l3.55 kg. of aluminum orthophosphate, 2.55 nucleus phase kg. of zirconium silicate, 2.91 kg. of TiO,, 0.8l kg. of arsenic and L65 kg. of sodium nitrate are homogenized for about 30 70 minutes in an ordinary commercial mixer. The mixture is If slower rates of heating are used to convert a compact melted down in a ceramic pot at a melting temperature of glass casting of the composition of the invention into the vitrezihout l.600 C for l2 hrs. and refined at this temperature for ouscrystalline state due to the greater thickness of the 0h 22-24 hrs. Glass articles are produced from this melt by jects, the holding time for the development of crystallization nuclei can be done away with.

pressing or blowing or drawing or rolling or casting and finally EXAMPLE 3 m, 25 no, 2 2

A cube of glass having the composition of Example 2 and an l 1 0 S edge length of 100 mm. was heated at a rate of 4 C. per hour, M30 0 7 to 830 C. and then allowed to cool by disconnecting the elec 5 7 A l t l i trically heated furnace. The properties of the resultant trans- .g.ass.com posmon as amed m claim I havmg the parent, vitreouscrystalline material corresponded to the pro- Composmon m welgh percent perties of a glass ceramic of the same chemical composition which had been converted in accordance with the heating proif; gram described in Example 2 (Table 6). p 5 l 7 We claim: Li,0 3.9 Na,0 o 6 1. Glass, having a working temperature in the range of M30 Asp-i D 5 l,l9() C. to l,320 C. [or a viscosity of i0 poises convertible by heat treatment to, transparent glass ceramic, consisting essentially ofin weight percent:

8. A glass composition as claimed in claim I having the composition in weight percent:

. sio, 51.3 ZnO z n i' ,\|,o 24 J Tio, 4 a 5-H 6 R H15 5 Li,0 3.4 N'.|,O u 6 i 04 M80 M L0 ASA), 0 5 0-5 Zno e i t l s-e TiO, 9. A lass com sition as claimed in claim I havin the I I 0 g 1 composition in weight percent:

04) 6 mp o 5-! As,O,

sio, 40.7 ZnO Al o 23.8 T o i U and wherein the ratio of P 0 to TiO is up to about 4, and the 5 6 8 sum of TiO and ZrO is at least about 3 weight percent, at up 3.8 Na,O 0 5 least about 90 weight percent of the composition correspond M80 I l 1 0 5 ing to B-quartz solid solution.

2. A glass composition as claimed in claim 1 having the 30 composition in weight percent:

10. A glass composition as claimed in claim I having the composition in weight percent:

57-60 SiO, i 54 n ZnO l 460 ZnO 2 2 M i z 0-] u 1.0, 1 28.8 no, 3 1 1 I 1-2 0 2,0, 1 1L6 I a 1 6 M 0.54 0 A5 0. 1 3.8 Mtg U 5 s 0 5-u rt w o a ii M101 0 5 11. A glass composition as claimed in claim I having the 3. A glass composition as claimed in claim 1 having the V composition in weight percent:

composition in weight percent:

40 Sin, n i ii no z 4 :5 ZnO I it A|,o 2i ll no, i s 10 265 no, 4 i, '3 5 3 ZrO, 2 1 1 3 7 Zro, u up 2 7 w o mm- U10 (I 5 M50 1 2 As,0, 0 e. 10 0.9 A .O. (I 5 45 I 4. A glass composition as claimed in claim I having the B composmon as Claimed Cla'm I havmg the composition in weight percent: P II H n eight percent:

5'0, 00 7 Zn() i x i 1 ZnO 4 4 Al,0 i9 7 1'10, 4 6 M10, 295 Tit), 4 (7 P,0\ 6 3 2:0, I 9 5.3 210, l 9 Li,0 J Na,O 0 5 M 1-2 Na.o 0 5 M50 0 o ,0, 0 5 50 2 2 ,0, 0 s S. A glass composition as claimed in claim 1 having the ll A glass conjlposluon clalifled in claim t and Consistcomposition i i h pemem; 5 mg essentially of. in percent by weight:

S10, 57 Q Z O i 7 52-56 g g AI,O, 211 712 24 8 mp P,O, a 0 2m, i i 7 P,0, 3 7-4 L. O t o 2 9 M 0 u s 1 M50 2 1 A510, on 094 I MgO 2: s zno 2 m2 9 no, 6. A glass composition as claimed in claim I having the I Q be composition in weight percent. ii 541 1 AMO 0 541 6 w o so, 55 1 no t s UNITED STA'IES PATENT OFFICE CERTIFICA'IE 0F CORRECTION Patent 3,642,504 Dated February 15, 1972 Inventor(s) JURGEN PET OLD Et- A].-

It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Col. 2, the table, MgO, No. 8, change "1.0" to --1.1--.

Col. 3, Table 2, left column, change the line reading "Properties of transparent galss" to --Properties of transparem glass ceramic--; and change the next line to read 0: 20-300.

Signed and sealed this 2nd day of January 1973.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents 

2. A glass composition as claimed in claim 1 having the composition in weight percent: 57-60 SiO21.5-2.0 ZnO 21-24 Al2O3 2.0-3.0 TiO2 6-8 P2O5 1.7-2.0 ZrO2 3.3-3.6 Li2O 0.5-1.0 As2O3 0.5-0.9 MgO 0.5-0.6 Na2O.
 3. A glass composition as claimed in claim 1 having the composition in weight percent: SiO263.0 ZnO 2.4 Al2O3 21.0 TiO2 1.5 P2O5 5.3 ZrO2 2.3 Li2O 2.7 Na2O trace MgO 1.2 As2O3 0.6.
 4. A glass composition as claimed in claim 1 having the composition in weight percent: SiO2 60.7 ZnO 1.8 Al2O3 19.7 TiO2 4.6 P2O5 6.3 ZrO2 1.9 Li2O 3.1 Na2O 0.5 MgO 0.9 As2O3 0.5.
 5. A glass composition as claimed in claim 1 having the composition in weight percEnt: SiO257.4 ZnO 1.7 Al2O3 23.3 TiO2 3.2 P2O5 6.0 ZrO2 1.7 Li2O 2.9 Na2O 0.5 MgO 2.7 As2O3 0.6.
 6. A glass composition as claimed in claim 1 having the composition in weight percent: SiO2 55.3 ZnO 1.5 Al2O3 25.0 TiO2 2.2 P2O5 7.9 ZrO2 1.9 Li2O 4.5 Na2O 0.5 MgO 0.5 As2O3 0.7.
 7. A glass composition as claimed in claim 1 having the composition in weight percent: SiO253.2 ZnO 2.2 Al2O3 26.3 TiO2 2.6 P2O5 7.8 ZrO2 1.7 Li2O 3.9 Na2O 0.6 MgO 1.2 As2O3 0.5.
 8. A glass composition as claimed in claim 1 having the composition in weight percent: SiO2 51.3 ZnO 2.0 Al2O3 24.3 TiO2 4.6 P2O5 10.5 ZrO2 1.8 Li2O 3.4 Na2O 0.6 MgO 1.0 As2O3 0.5.
 9. A glass composition as claimed in claim 1 having the composition in weight percent: SiO2 46.7 ZnO 2.2 Al2O3 28.8 TiO2 3.0 P2O5 11.6 ZrO2 1.8 Li2O 3.8Na2O 0.5 MgO 1.1 As2O3 0.5.
 10. A glass composition as claimed in claim 1 having the composition in weight percent: SiO2 46.0 ZnO 2.2 Al2O3 28.8 TiO2 3.7 P2O5 11.6 ZrO2 1.8 Li2O 3.8 Na2O 0.5 MgO 1.1 As2O3 0.5.
 11. A glass composition as claimed in claim 1 having the composition in weight percent: SiO2 44.5 ZnO 1.8 Al2O3 26.5 TiO2 4.6 P2O5 15.7 ZrO2 1.9 Li2O 3.1 Na2O 0.5 MgO 0.9 As2O3 0.5.
 12. A glass composition as claimed in claim 1 having the composition in weight percent: SiO2 38.9 ZnO 4.4 Al2O3 29.5 TiO2 4.6 P2O5 15.3 ZrO2 1.9 Li2O 2.2 Na2O 0.5 MgO 2.2 As2O3 0.5.
 13. A glass composition as claimed in claim 1, and consisting essentially of, in percent by weight: 52-56 SiO2 24.5-28 Al2O 7.5-9 P2O5 3.7-4 Li2O 0.9-1.1 MgO 2-2.5 ZnO 2.6-2.9 TiO2 1.7-1.9 ZrO2 0.5-0.7 As2O3 0.5-0.6 Na2O. 